In medical imaging so-called hybrid modalities, such as PET-CT, SPECT-CT, PET-MR and SPECT-MR for example, are becoming increasingly important. The meanings of these abbreviations are as follows:    PET: Positron Emission Tomography    CT: Computed Tomography    SPECT: Single Photon Emission Computed Tomography    MR: Magnet Resonance Tomography
The advantage of these combinations is the connection of a modality with a high local resolution (especially MR or CT) with a modality with high sensitivity (especially SPECT or PET). The description below refers to a combined PET-MR system. Embodiments of the present invention can however be generally transferred to all forms of hybrid modalities or to measurement methods associated therewith.
PET uses the particular properties of positron emitters and positron annihilation in order to determine quantitatively the function of organs or cell areas. In such cases the appropriate radio pharmaceuticals which are marked with radio nuclides are administered to the patient. As they decay, the radio nuclides emit positrons which after a short distance interact with an electron, which causes what is referred to as an annihilation to occur. During this process two Gamma quanta occur which fly off in opposite directions (displaced by 180°). The Gamma quanta are detected by two opposite PET detector modules within a specific time window (coincidence measurement), by which the location of the annihilation is determined to a position on the connecting line between these two detector modules.
For verification the detector module must generally cover a large part of the gantry arc length for PET. It is divided up into detector elements with sides of a few millimeters in length in each case. On detection of a Gamma quantum each detector element generates an event recording which specifies the time as a well as the verification location, i.e. the corresponding detector element. This information is transferred to a fast logic and compared. If two events coincide in a maximum time interval then it is assumed that a Gamma decay process is occurring on the connecting line between the two corresponding detector elements. The PET image is reconstructed with a tomography algorithm, known as the back projection.
Since MR systems operate with high magnetic fields, the use of materials within these systems which are compatible with said fields is necessary. Attention should be paid in particular in the construction of PET detectors in combined PET-MR systems to the insensitivity of the detectors to magnetic fields.
In US 2007/0102641 A1 a combined PET-MR system is described in which lutetium oxyorthosilicate (LSO) is used as scintillation crystal for converting the Gamma quanta into light and Avalanche Photo Diodes (APD) are used for detection of the light. The APDs are connected with pre-amplifiers. A ring of such PET detectors is arranged within an MR device. This allows MR and PET data sets to be recorded simultaneously. A comparable arrangement is known from U.S. Pat. No. 7,218,112 B2.
With the frequently used semiconductor amplifiers and semiconductor detectors (Avalanche Photo Diodes, APD) in particular the amplification is dependent on the temperature. Since the components are subjected to temperature variations during operation, a cooling is necessary. Feeding in cooled air allows the temperature of the amplifier and photodiodes to be regulated. When air at a constant temperature is used the temperature of the amplifier is a result of the equilibrium of the generated heat and the heat emitted through the air over the surfaces of the amplifier. The cooling can be used in the same way for other parts of the detection system.
The APDs are however not only subjected to temperature fluctuations because of their operation. In particular the proximity to the gradient coil and the excitation coil of the MR system caused by the compact design represents a heat source acting from outside on the APD. The temperature of a gradient coil is typically between 20 and 80° C. during operation. These temperature differences also affect the APDs and thereby their amplification. The effects of this heat source can only be overcome with difficulty using air cooling. It is therefore of advantage to provide water cooling.
In addition to the problem of cooling, PET detectors in particular are very sensitive to faults caused by electromagnetic fields. The very small currents measured with time resolution are typically responsible for these faults. Integration into the MR system then subjects the PET detector to the fields which are necessary for imaging by way of MR. Gradient fields which are driven by an amplifier operating in accordance with the switched-mode converter principle cause faults in such cases of frequencies ranging up to a few 100 kHz. The APD is to be shielded from these faults. In addition the HF system of the MR device can generate higher frequencies which are also be taken into consideration.
It is known that electro magnetically-sensitive components can be shielded by copper foil against electromagnetic faults for example. The shielding effect is produced by currents being induced in the shielding material, which then cancel out the electromagnetic fields inside the shielding. The thickness and the geometry of the shielding structure define the frequency range in which the shielding is effective.
It is thus possible to provide a PET detector block with a shielding envelope. Since however the heating of the shielding caused by the switching of the gradients increases in proportion to the surface, it is desirable to embody the shielding small if possible in order not to introduce any unnecessary heat into the PET detection system.
In at least one embodiment of the present invention, an improved detection device with optimized shielding is provided.
In accordance with one version of at least one embodiment of the invention, a detection device with at least one detector and a processing unit for processing signals of the detector is specified, with at least one cooling unit for cooling the detector and processing unit being provided. Electromagnetic shielding is provided for the detector and the processing unit which comprises that least two electrically-connected sections of which are first section has a higher electrical conductivity than a second section and with the second section being in thermal contact with the cooling unit. The detector and the processing unit for the signals of the detector can be efficiently cooled by the cooling unit provided.
The fact that the shielding comprises two sections with different levels of electrical conductivity means that the power dissipation produced by the shielding falls primarily on the section with the low electrical conductivity. However this section is in contact with the cooling unit so that the heat arising is able to be removed efficiently. Both the detector and also the processing unit are consequently not affected by the heating up of the shielding so that the shielding can be arranged very close to the processing unit and the detector. This means that the shielding is able to be made a significantly smaller by comparison with a complete encapsulation of the PET detector block.
A different electrical conductivity means that the electrical resistance (e.g. per unit of surface) of the shielding is different in different sections. A low electrical conductivity means in such cases that the electrical resistance is increased by comparison with the section with the higher electrical conductivity.
In an advantageous embodiment of the invention the shielding only partly surrounds the detector and the processing unit. This typically makes the detector accessible for optical signals. It should be ensured that the cutout areas of the shielding do not adversely affect its shielding effect in the relevant frequency range.
In an advantageous embodiment of the invention, the detector comprises a light generator and a light sensor, with the light sensor being arranged such that it converts light of the light generator into an electrical signal and can transmit it to the processing unit. For example Gamma quanta can be converted by the light generator into visible light and then by way of the light sensor into an electrical signal by way of such a detector.
One embodiment of the invention is advantageous in that the first section of the shielding is arranged between the light generator and the light sensor and features at least one cutout which is arranged such that the light of the light generator can reach the light sensor. The cutout makes it possible for light from the light generator to get through the shielding to the light sensor.
In an advantageous embodiment of the invention the first section of the shielding features an interruption assigned to the cutout which is embodied such that a current path surrounding the cutout is electrically interrupted. This effectively prevents in the case of incident electromagnetic radiation around the cutout eddy currents being generated which generate a heating-up in the non-cooled area of the shielding.